1. Field of the Invention
The present invention is related to an organic electroluminescence element, a method for manufacturing the element and an organic electroluminescence display device. In particular, the present invention is related to an organic electroluminescence element, a method for manufacturing the element and an organic electroluminescence display device, in which the element and the display utilize electroluminescence (EL) phenomenon of an organic thin film.
2. Description of the Related Art
An organic EL element has an organic light emitting layer presenting an electroluminescent phenomenon between an anode and a cathode. When a voltage is applied between the electrodes, a hole and electrons are injected into an organic light emitting layer. Then, the hole and electrons recombine in the organic light emitting layer and the organic light emitting layer emits light. In other words, an organic electroluminescent element is a self-luminous element.
For the purpose of increasing light emitting efficiency, a hole injection layer, a hole transport layer and an electron blocking (an interlayer layer) are appropriately selected and arranged between an anode and an organic light emitting layer, and, a hole blocking layer, an electron transport layer and an electron injection layer are appropriately selected and arranged between an organic light emitting layer and a cathode. Here, an organic light emitting layer as well as a hole injection layer, a hole transport layer, an electron blocking layer (an interlayer layer), a hole blocking layer, an electron transport layer, an electron injection layer are called a light emitting medium layer.
A low molecular type material or a high molecular type material, both including mainly an organic material, and an inorganic material are used for the respective layers of the light emitting medium layer.
Examples of low molecular materials are described below. Copper phthalocyanine (CuPc) is used as a hole injection layer. N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) is used as a hole transport layer. Tris (8-quinolinol) aluminium (Alq3) is used as an organic light emitting layer. 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4,-oxadiazole (PBD) is used as an electron transport layer. Lithium fluoride (LiF) is used as an electron injection layer.
Generally, the thickness of each layer of the organic light emitting medium layer including a low molecular material is from 0.1 to 200 nm. These layers are formed by a dry process in a vacuum condition such as a vacuum evaporation method such as a resistance heating method or a sputter method.
In addition, there are various kinds of low molecular materials. An improvement of luminous efficiency, emission brightness, life time or the like by combining the various materials is expected.
On the other hand, for example, for polymeric materials, the following materials for an organic light emitting layer can be used: The material which low-molecular luminescent coloring matter dissolves in polymers such as polystyrene, polymethyl methacrylate and polyvinyl carbazole; Macromolecular fluorescent substance such as polyphenylene vinylene derivative (PPV) or poly alkylfluorene derivative (PAF); and Polymer phosphor such as a rare earth metal system.
Generally a coating liquid (an ink) is made by dissolving or dispersing these polymeric materials in a solvent. Further, using a wet process such as application or printing, a layer of which thickness is about 1-100 nm is formed.
In comparison with a dry process in a vacuum condition such as a vacuum evaporation method, a wet process has the following merits: Film formation under atmospheric air is possible; facilities are inexpensive; upsizing is easy; and a layer can be formed efficiently in a short time.
In addition, an organic thin film layered using a polymeric (high molecular) material has the following merits: Crystallization and cohesion are hard to occur; and a pinhole and a foreign matter of the other layer can be coated. Therefore, an organic thin film made of a polymeric material can prevent defects such as a short-circuit or a dark spot.
Examples of inorganic materials include alkali metals such as Li, Na, K, Rb, Ce and Fr, alkaline-earth metals such as Mg, Ca, Sr and Ba, lanthanoids such as La, Ce, Sm, Eu, Gd and Yb, metallic elements such as Au, Cu, Al, Fe, Ni, Ru, Sn, Pb, Cr, Ir, Nb, Pt, W, Mo, Ta, Pa and Co, Si, Ge, and oxide, carbide, nitride, and boride thereof.
There are a lot of kinds of materials of a high adhesiveness and a high stability to heat among an inorganic material, compared with organic materials. Therefore, if an inorganic material is used, it is expected that occurrence of non-light emitting area called dark spots is reduced, and that light emitting characteristic and life time are improved. In addition, an inorganic material is relatively inexpensive compared with an organic material. If application to a large-sized display or high-volume production is considered, an inorganic material plays an important role in cost reduction.
Patent document 1 discloses a structure in which an inorganic hole injection layer using an inorganic material is formed between an organic light emitting layer and an anode which is an hole injection electrode. The structure utilizes the above feature of an inorganic material.
In addition, patent document 2 and patent document 3 disclose a structure. In the structure, an inorganic oxide electron injection layer is formed between an organic light emitting layer and an anode which is an electron injection electrode.
It is known that molybdenum oxide is especially useful from the following reasons: film formation is easy; and a hole injection function or a hole transport function from a hole injection electrode is relatively high compared with other inorganic materials.
In general, a hole injection transport function is determined by the number of carrier, the mobility of carrier or both of them. Resistivity and a hole injection function depends on IP (ionization potential) etc. An inorganic material as molybdenum oxide has usually a high resistivety. There is a difference in IP between an inorganic material (molybdenum oxide) and an anode. A hole injection function and a hole transport function of an inorganic material (molybdenum oxide is insufficiently yet. Current is difficult to flow. A sufficient light emitting luminance can not be obtained. In addition, holes sufficient for electrons are not injected to a light emitting layer. That is, carrier balance is lost.
The present invention includes a flat molybdenum oxide including a crystal structure. Therefore, resistivity or ionization potential is optimized. Injection property or transport property of carrier is improved. The present invention provides an organic electroluminescence element, a method for manufacturing the element and an organic electroluminescence display device, in which light emitting luminance and life time are high and there was no defect.
[patent document 1] JP-A-H11-307259
[patent document 2] JP-B-3692844
[non-patent document 1] JP-B-3773423